1. Field of the Invention
This invention relates to the deactivation of magnetic targets; and in particular it concerns novel methods and apparatus for deactivating targets used to protect merchandise from shoplifting or theft.
2. Description of the Prior Art
Various types of electronic equipment are used to protect against shoplifting or theft of merchandise. One such type, which is described in U.S. Pat. No. 4,623,877 is generally referred to as a "magnetic" type of electronic article surveillance equipment. This magnetic type equipment makes use of "targets" which are thin elongated strips of high magnetic permeability, low coercivity material (e.g. Permalloy metal and certain amorphous alloys of iron, nickel or cobalt). These targets are fastened to articles to be protected. An interrogation antenna is energized to generate a continuous alternating magnetic interrogation field at each exitway from the protected area; and when a protected article is carried through the exitway it becomes exposed to the magnetic interrogation field and is driven successively into and out of magnetic saturation by the alternating magnetic interrogation field. This results in a disturbance of the interrogation field such that other magnetic fields are produced at frequencies harmonically related to the interrogation field. There is also provided at the exitway a receiver antenna and a receiver which detect these other fields and produce an alarm when a protected article is carried out through the exitway.
In order to authorize passage of an article from the protected area so that it does not cause an alarm, the target must be deactivated. One means for deactivating magnetic targets is shown and described in U.S. Pat. Nos. 4,665,387 and 4,684,930. As there shown, the target is provided with a long continuous strip of a magnetically "hard" material, i.e. a material that has sufficiently high magnetic coercivity that when it becomes magnetized by externally applied magnetic fields of sufficient intensity, it retains its magnetization. When this strip, which is known as a "collinear strip", is magnetized according to a particular pattern along its length, it prevents its associated target from producing magnetic fields at harmonic frequencies and thereby effectively deactivates the target. A problem that is encountered with the use of collinear deactivating strips is that in order to magnetize such strips the source of the magnetization must be of the particular pattern, i.e. a series of spaced north and south poles, and the source of magnetization must be placed very close to the target. Thus remote deactivation is not practical using this type of deactivation.
Other means for deactivating magnetic targets are shown and described in U.S. Pat. Nos. 3,820,103, 3,820,104 and 3,765,007. According to these patents, a magnetic target is provided with a series of spaced apart elongated elements, generally referred to as "slugs", which are shorter than the target itself and which are spaced apart from each other along the length of the target. These slugs are also of high magnetic coercivity. When the target is aligned with an external magnetic field, each of these slugs becomes magnetized along its length with each slug possessing its own north and south poles. These separate and spaced apart pole pairs magnetically bias the target so that it cannot respond effectively to interrogation fields.
The use of spaced apart magnetizable slugs permits target deactivation to be carried out with a single magnetizing field from a magnetizing device that does not touch the target. However, it necessary that the target be aligned with the deactivating magnetic field. This makes it difficult to deactivate targets on several articles of merchandise that may be in random orientation on a checkout conveyor or in a bag or box.
It has been proposed to deactivate magnetic targets which have spaced apart deactivating slugs by providing a box-like structure with magnetizing coils in various walls of the structure. When the coils are energized by passing an electric current through them, they will produce magnetic fields according to their different orientations. However, when magnetic fields are generated from coils facing in different directions, there are produced resultant fields which are different in intensity and direction at different locations. Thus unless a target at a particular location is oriented in line with or close to the direction of the resultant field at that location, the target will not be effectively deactivated.